This invention relates to the field of engines, specifically to a four-cycle, four cylinder, free-piston, premixed charge compression ignition, internal combustion reciprocating piston engine with a variable piston stroke that offers the potential of enhanced efficiency, lower emissions, and multi-fuel operation. Applications include but are not limited to, use with automotive vehicles, engine driven pumps, engine driven compressors, small aircraft, marine vehicles, and power tools.
Development of the Premixed Charge Compression Ignition (PCCI) engine, and related Homogeneous Charge Compression Ignition (HCCI) internal combustion reciprocating engine has been ongoing for a number of years at Companies, Universities and U.S. National Laboratories—as well as numerous foreign institutions. Among these are:
Companies:
    Caterpillar Inc.    Cummins Inc.    General Motors Corporation    Waukesha EngineUniversities:    Helsinki University of Technology, Helsinki Finland    Hokkaido University, Sapporo Japan    Lund Institute of Technology, Lund Sweden    Massachusetts Institute of Technology, Boston Mass.    Norwegian University of Science and Technology, Trondheim Norway    University of California, Berkley Calif.    University of Michigan, Ann Arbor Mich.    University of Minnesota, Minneapolis Minn.    University of Wisconsin, Madison Wis.Laboratories:    Argonne National Laboratory    Lawrence Livermore National Laboratory    National Energy Technology Laboratory    Oak Ridge National Laboratory    Sandia National Laboratory
The PCCI/HCCI engine offers the potential of higher efficiency, lower emissions and multi-fuel operation. The technology is scalable from watts to megawatts. Development has progressed on both two and four-cycle versions of PCCI/HCCI engines. Notable work in this field includes that of: Cummins Inc. on four-cycle PCCI engines, resulting in U.S. patent application 2004/00103860; Van Blarigan at Sandia National Laboratories, on the two-cycle, free piston, HCCI “Thermodynamic Fuel Cell” with U.S. Pat. No. 6,199,519 B1; and Caterpillar Inc. with a two-cycle free piston engine with hydraulic output as disclosed in U.S. Pat. No. 6,463,895 B2. However, these efforts have not yet produced a truly viable PCCI/HCCI engine. In four-cycle PCCI/HCCI engines, derived from conventional engines, e.g. Cummins, the fixed stroke of the pistons results in a fixed compression ratio that is a constraint on charge ignition. Thus, this PCCI/HCCI four-cycle engine work, to-date, is based primarily on modification of existing four-cycle engines, with emphasis on control of the charge parameters. These modifications have focused on the very complex control of the individual cylinder charge composition and temperature required to properly time the charge ignition, so as to attain acceptable combustion uniformity between cylinders. The resulting four-cycle engines are highly complex and are very sensitive to ambient conditions and fuel properties. Sensitivity is such that the previously mentioned Cummins published U.S. patent application focuses on the use of duel fuels to modify the auto-ignition properties of the charge. Despite these current difficulties, a study by TIAX, a product and technology development firm, and Global Insight, an industry forecasting firm, titled “The Future of Heavy-Duty Powertrains”, predicts “HCCI engines will power nearly 40% of heavy-duty vehicles by 2020.—Initially HCCI will only be able to power light loads at low speeds so early versions of the engine will also incorporate conventional diesel combustion to supply more power when greater demand is placed on the engine.” The study also predicts “—a full mode HCCI engine will eventually supersede the initial mixed mode HCCI/diesel technology.”
To avoid the complexities of modified conventional engines noted in the foregoing, researchers have investigated other engine configurations for PCCI/HCCI. Free piston engines inherently have a variable stroke and thus avoid the constraint encountered in conventional crankshaft engines. Considerable work has been done, over many decades, in an attempt to develop a practical free piston engine. However, success to date has been limited. Most of this prior effort focused on the two-cycle Diesel variant, with less work on spark ignited engines. The recent work by INNAS Free Piston B.V. to produce a free piston single cylinder engine with hydraulic power output is an example of efforts to produce a practical two-cycle Diesel, free piston engine, see U.S. Pat. No. 6,279,517 B1. Also, Sunpower, Inc., as disclosed in U.S. Pat. Nos. 5,775,273 and 6,035,637, proposes a spark ignited free piston engine design with variable valve actuation and in which the expansion stroke is greater than the compression stroke (for increased efficiency). Pembek Systems Pty Ltd., Australia, advocates “The Free Piston Power Pack” for hybrid electric vehicles utilizes multiple units of two opposed linear piston, two-cycle, free piston engines (Diesel or spark ignited) that have integral linear generators and are self scavenging, see U.S. Pat. No. 6,651,599 B2. However, to date, none of the foregoing has demonstrated a substantive improvement in two-cycle free piston engine performance that would provide a PCCI/HCCI modification to that technology offering improvement over the PCCI/HCCI modified conventional engine. Recent analytical efforts, such as that of Van Blarigan at Sandia, have utilized the PCCI/HCCI cycle in a free piston engine and confirm that conclusion. The two-cycle PCCI/HCCI engine under development at Sandia is more limited in speed range and throttling, produces higher emissions, has lower energy density (primarily due to the linear alternator) and greater fuel consumption (primarily due to charge scavenging limitations inherent in two-cycle engines) compared to a like four-cycle PCCI/HCCI engine. Lotus Engineering Ltd. in conjunction with the University of Sheffield and the University of Loughborough in the U.K. are researching a two cylinder four-cycle free piston linear engine with a integral linear alternator (similar in geometry to the Sandia two-cycle unit) in which the alternator output would be stored as electrical energy (in an external storage device) during the power/exhaust strokes and then used to run the alternator as a motor to drive the piston(s) during the intake/compression and exhaust strokes. (See presentation, “Four Stroke Free Piston Energy Converter” made at the Fuel Cell and Battery Vehicle Industry Academic Network (FABIAN), April 2005 MIRA conference at the University of Sheffield, U.K. available at http://www.shef.ac.uk/fabian/stewart_ws5.ppt) Their preliminary work indicates that this engine offers many improvements over the two-cycle free piston engine, including the use of PCCI/HCCI combustion. However, the proposed engine is complex, requiring a linear alternator/motor with sophisticated and costly energy conversion circuitry and external energy storage. Further, the energy density of the engine and combined supporting devices will be low.
Of further noteworthiness is the recent effort by Kvaerner ASA to develop a Diesel two-cycle free piston gas generator with a power turbine output. (See “Dynamics and Control of a Free piston Diesel Engine” by Johansen et all, Norwegian University of Science and Technology, Department of Engineering Cybernetics, Trondheim, Norway and Kvaerner ASA Technology Development, Lysaker, Norway available at http://citeseer.csail.mit.edu/601185.html.) This effort utilizes technology originally by Pescara and disclosed in U.S. Pat. Nos. 1,657,641, 2,162,967 and 2,581,600, in 1925, 1935 and 1941 respectively, and then furthered by GM, Ford, Junkers and others from the 1930's through the 1960's. It is also related to the subject matter disclosed in U.S. Pat. No. 4,873,822 to Benaroya (1989), entitled “Energy Producing Installation with Internal Combustion Engine and Turbine”. The objective of the Kvaerner effort is to produce an engine with a rating of 8 MW having the low weight and compactness of the gas turbine and the low fuel consumption (50% efficiency) of the Diesel engine for Marine propulsion applications. Initial results from a single cylinder test bed engine are promising.
The complexities and limitations of the previously cited engines can be overcome by a four cylinder, four-cycle, free piston, PCCI/HCCI engine disclosed herein, which provides a four cylinder, four-cycle, Free Piston Floating Stroke (FPFS), PCCI/HCCI internal combustion, reciprocating piston engine. Hereinafter, the present invention will be identified by one or more of the following terms, the FPFS engine, the present invention, and the present FPFS engine.
The FPFS engine disclosed herein also includes a gas generator/power turbine configuration, as shown in FIGS. 16-22,—which would retain the advantages of the foregoing mentioned Kvaerner power turbine two-cycle free piston engine but have the lower emissions and lower fuel consumption of a four-cycle PCCI/HCCI engine.
The free piston engine, while solving the charge combustion timing problem associated with crankshaft PCCI/HCCI engines, does not provide a means of directly producing rotary power output. The FPFS engine disclosed herein addresses the foregoing issue by employing a variety of mechanisms to directly utilize the linear motion of the free piston engine or to indirectly convert it to rotary motion.
A four-cycle configuration of a FPFS PCCI/HCCI engine benefits greatly from variable valve actuation (VVA)—to the degree that VVA becomes a practical necessity. Several variable valve geometries are currently under development by others, notable among these are: Sturman Industries, Inc., U.S. Pat. No. 6,820,856 to Grill (2004); Massachusetts Institute of Technology Laboratory for Electronics and Electromagnetic Systems electromagnetic valve drive system, (see the MIT article “Design and Experimental Evaluation of An Electromechanical Engine Valve Drive” published for the 2004 35th Annual IEEE Power Electronics Specialists Conference and available at:
http://www.mit.edu/˜djperrea/Publications/Conference%20Papers/cpPESC04p4838.pdf); and Johnson Controls (see presentation “Electromechanical Valve Actuation” at the MIRA conference at the University of Sheffield, Sheffield England Oct. 13, 2004 available at: http://www.shef.ac.uk/fabian/mareky_ws4pdf.pdf.) Nonetheless, disclosed herein is a VVA mechanism, as shown in FIGS. 11A, B, C, & D, that offers similar performance to those being developed by the foregoing, with the prospect of a lower cost of implementation than from those sources.
There are numerous efforts to improve existing engine technology to lower emissions and improve efficiency e.g. ARES, ARICE, Freedom Car, Advanced Heavy Hybrid, 21st Century Truck Program, etc. Further, there are long term efforts to increase Diesel engine efficiency by Increasing peak pressure. In a study by TEKES ProMOTOR, at the Helsinki University of Finland, extreme engine operating parameters were investigated, including very high operating pressures. (See “Extreme values of the piston engine—Final Report”, Sep. 30, 2003 by TEKES ProMOTOR, Academy of Finland, TUKEVA, available at http://www.icel.tkk.fi/eve/ICEL_Final_report.pdf.) It is well understood that the efficiency of gas power cycles in engines is primarily related to the engine mean effective pressure (higher pressure yields higher efficiency), which in turn is limited by engine component design and the materials then available, key among them being the crankshaft. They cite in the foregoing study, as one of the limiting factors to further engine development at these extreme conditions, the inability of the crankshaft to carry the increased loads from such high pressure operation. The crankshaft size required by such loads increases significantly, and the crankshaft size ultimately remains a limiting factor to further increasing engine operating pressure. By using an engine design, such as that being developed by Kaeverner, or as disclosed herein, which utilizes an indirect power extraction method such as the power turbine, the limitations of the crankshaft are circumvented. Note: In the design herein disclosed, as shown in FIGS. 16-22, the piston connecting elements, 1, 24, 25, 26, 45 & 46, do not carry the output load, but instead carry the much lower load required by the pressure of the cylinder then in the compression stroke (and pumping load of the exhaust stroke). This represents a significant advantage toward improving engine efficiency as higher operating pressures may be more readily attainable.